Polydactylism in cats is a genetic mutation, and a dominant gene. The mutation appears to have originated in Kent in the 1600s, and polydactyl cats were brought to North America by the Puritans in the 1630s. They were thought to be a sign of good luck to ship captains. Polydactylism is a defining characteristic of the Maine Coon cat, though about forty percent of Maine Coons have the normal compliment of toes. 
Polydactyls are most common in the Boston and Philadelphia areas, not very common in New York, and unknown in Europe.
I had a cat with seven toes on her front paws, six on her back. The extra toes she could work like thumbs, and she often amused herself by playing with marbles–she could pick them up just by wrapping her thumbs around them, and then hide them under kitchen mats.
She took ill and died quite suddenly one day. Some disease, something passed through. It killed off most of the feral barn cats in the area, too.
She had died in my arms. Burying her was hard. I bought a bag of marbles at the hobby shop, and put them and some of her cat toys in the shoebox with her. It was hard.
Hi. I adopted a cat from an animal shelter (actually four and all neutered). One is a “poly”. He has 8 toes on R front paw, 7 on L front paw, and a dew claw on his back paw. It looks like he has a paw and a half on each front foot. He is known as Bigfoot (Biggy for short). He was a stray, all skinny and scraggly, but he looks absolutely handsome now. He’s long hair orange and white about a year old. All four boys get along so good, they’re all orange and white too. I love my boys. They are spoiled. They each take a turn going for rides with us in the car. It’s only if we go through a drive through for snacks, never ever left in the car alone. I do have a question. Do cats like Biggy have problems with arthritis in their feet as they get older or have any trouble walking at an older age? Is there a website where I can see an Xray of a poly cat’s foot? Thank you. Betty 8/12/08.